The argument of "Mel bypassed the DK's borrowed magic" so he must be far stronger than anyone else doesn't even make any sense to begin with. In chapter 249/250, Ludo casually bypassed that same magic while being on a human vessel, while later on he couldn't do it with stronger attacks.
There are multiple takes on this:
1. Zeldris was up against one of the most overpowered magical users in the series (Merlin) and someone who he himself was very wary about, so he decided to deactivate the DK's borrowed magic. Makes no sense as you can see it.
2. Nakaba didn't had a clear idea as to what the DK's magic was supposed to be at that time. This is supported by the fact that it was highly implied during Zel's battle against Diane in Drole's body that the DK's magic had something to do with sealing magic. A point that hasn't ever been brought up again.
Again, the first option doesn't even make any sense, while the second option is much better and makes far more sense than that. This is not the first time Nakaba actually has retconned things he has said before. Besides, Zeldris himself stated in his fight against Mael that no matter how powerful the magic is, it can't bypass his DK borrowed magic (which contradicts several panels shown before and further supports the idea that Nakaba changed his mind during the series as to what the DK's magic was supposed to be). This is again supported by the fact that not even the DK's magic attack (Sighting Sage) was able to bypass it, and 10C DK Mel should be far above Prime Mel.
The only reason later on the DK could hurt him with a magical attack was because he took back the borrowed power he lent to Zeldris. The dialogue he said in the official translation while he did that was: "Did you forget? The Demon Lord power was something I'd only lent him", which clearly indicates that he had to take that power out of Zeldris for his magical attacks to affect him.
So, the "Mel bypassed the DK's magic" argument shouldn't even be brought up because it makes no sense and it doesn't help at all to guess how strong Prime Mel really is, since on the one hand Ludo in a human vessel could casually do that and later on, not even the DK himself could do it.